You don't need to vacuum your molds, but you do need to make sure you cure the mold under pressure. Basically what's happening is air trapped in the mold is getting crushed once you apply pressure thus causing disfigurement of the cast as the resin flows into the spaces created. To fix this issue you should cure your molds under the same pressure you intend to cast them under. If your silicone takes 8 hours to cure I recommend leaving it under pressure atleast 10 hours so you don't waste the silicone. If it's colder out double the cure time under pressure. And make sure people know it's under pressure or in a safe place where no one will mess with it. I like to throw a heavy blanket over the pot once is under pressure. My casts are very good, and I don't use vacuum at all.